title: Inhibitory actions of methionine-enkephalin and morphine on the cat carotid chemoreceptors.
abstract: 1 The effects of intracarotid injections of methionine-enkephalin (Met-enkephalin) and morphine on chemoreceptor activity recorded from the peripheral end of a sectioned carotid sinus nerve have been studied in cats anaesthetized with pentobarbitone. 2 Met-enkephalin caused a rapid, powerful, inhibition of spontaneous chemoreceptor discharge, the intensity and duration of which was dose-dependent. 3 Morphine was a less potent inhibitor of spontaneous chemoreceptor discharge, and the inhibition it evoked was rather variable and tended to be biphasic. Low doses of morphine caused a slight increase in discharge. 4 Naloxone (0.2 mg i.c.) slightly increased spontaneous discharge, greatly reduced the chemo-inhibition caused by morphine, and reduced the inhibitory effect of Met-enkephalin. A higher dose of naloxone (0.8 mg) caused a substantial reduction of the Met-enkephalin effect. 5 Chemo-excitation evoked by intracarotid injections of acetylcholine, CO2-saturated Locke solution, and sodium cyanide were only slightly and somewhat variably reduced following injections of Met-enkephalin, whereas the inhibitory effect of dopamine was potentiated. Following morphine administration, response to acetylcholine and sodium cyanide were reduced slightly, whereas those to CO2 and dopamine were potentiated. 6 Responses to acetylcholine and CO2 were slightly potentiated during infusion of Met-enkephalin (50 micrograms/min, i.c.) and the response to sodium cyanide was slightly reduced. 7 It is concluded that naloxone-sensitive opiate receptors are present in the cat carotid body; when activated they cause inhibition of spontaneous chemoreceptor discharge. The physiological role of these receptors and the identity of any endogenous ligand remains to be established.
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002245:Carbon Dioxide / Q000494:pharmacology; D002346:Carotid Sinus / Q000187:drug effects; D002415:Cats; D002628:Chemoreceptor Cells / Q000187:drug effects*; D003486:Cyanides / Q000494:pharmacology; D004298:Dopamine / Q000494:pharmacology; D004723:Endorphins / Q000494:pharmacology*; D004744:Enkephalin, Methionine; D004745:Enkephalins / Q000494:pharmacology*; D066298:In Vitro Techniques; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology
chemical_list: D003486:Cyanides; D004723:Endorphins; D004745:Enkephalins; D002245:Carbon Dioxide; D009270:Naloxone; D004744:Enkephalin, Methionine; D009020:Morphine; D000109:Acetylcholine; D004298:Dopamine